My acer suddenly shuts off by itself

Hi there, i really need some help finding out what exactly is wrong with my acer Aspire 4738G , intel core i3 ati radeon HD5470 grafic card and running windows 7 64 bit...i been using my laptop for 4 years now .my system always been in tipn top condition but recently my laptop is getting power cuts when i use it for more than 10 hours or more.if i try to turn it back on right away it sometimes will work for 30 minute or so and then suddenly shuts off again...i notice if use all cores to convert file or crack password it wont even last for 30 minute...i got my laptop to servis n was inform that my fan is working fine...i sent to two shops to identify the issue...result is cant detect why...the shop guy said it could be graffic card faulty but thats just his educated guess....can anybody advice me pls.i would be very grateful..i love my laptop n dont want to replace it...pls advise...tq